1. Field of the Invention
The invention relates to methods and apparatus for performing non-destructive evaluation (NDE) of fastening bolt distal ends and their mating nuts. The bolt distal ends include the bolt head and male threaded bolt tip. The corresponding female threaded nuts engage the male threaded bolt tip. The invention also relates to methods and apparatus for performing NDE of a mated, engaged threaded bolt tip and nut, including in situ NDE within apparatus. In more specific exemplary applications, embodiments of the invention relate to NDE of turbine engine turbine bolts. In embodiments described herein the NDE modality is phased array ultrasonic inspection in either contact or immersion inspection modes.
2. Description of the Prior Art
Machinery, such as turbine engines incorporate threaded fasteners that are subject to stress and strain forces during machinery operation. FIG. 1 shows an exemplary quarter sectional, perspective view of a known turbine engine rotor thru-bolt 20 threaded bolt tip 22 and reduced diameter bolt shank 24. The enlarged view portion shows male bolt threads 26 and fastener thread side faces 28 that are defined between the thread roots 30 and thread crest 32. The bolt tip 22 threads 26 are adapted for mating engagement with a female threaded nut. The opposite distal end of the bolt 20 defines a bolt head. Exemplary known bolt head 34 and bolt nut 36 are shown in FIG. 7. It is desirable to inspect fastener threads for defects prior initial service use and/or during subsequent maintenance prior to continued service use. Particularly it is difficult to inspect critical threads in bolt tips or mating nuts at equipment field sites—especially if the fasteners are in situ within the machinery, such as within turbine engine rotors.
Generally cracks in the fastener threaded side face 28 area are very difficult to detect with current methods, especially in field sites. Known methods require human hand held visual or ultrasonic inspection. These known inspection methods are subject to human error during the inspection process as well as variations in the inspection quality by different service personnel. These variations in inspection processes increase potential for unnoticed or missed defects, as well as false identification of non-existent defects. Hand held ultrasonic inspection methods by holding an inspection probe relative to the fastener do not insure consistent scan angles, uniform pressure contact, or signal uniformity as the probe is maneuvered around the fastener. Lack of uniform pressure and consistent orientation during scan maneuvering causes scan signal variations that are not attributable to the fastener physical structure. Contact-type ultrasonic inspection is not successful if the fastener surface is rough or inconsistent. Additionally hand held ultrasonic inspection probes do not assure consistent physical maneuvering of the probe about the fastener or provide probe positional information (e.g., by a position encoder) that can be paired with the scan data for potential fastener 3-D modeling or archiving of scan defect/positional data. The aforementioned challenges to performing NDE ultrasonic inspection of bolt/nut fasteners are even more challenging when attempting to perform an in situ inspection of fasteners installed in machinery, such as turbine engine rotors.